Their wings,
especially, are useful to the perfect forms in finding one another, and
to the imperfect ones in migrating from one plant to its nearest
neighbours, where they soon become the parents of fresh hordes in rapid
succession. Hence various kinds of aphides are among the most dreaded
plagues of agriculturists. The 'fly,' which Kentish farmers know so well
on hops, is an aphis specialised for that particular bine; and, when
once it appears in the gardens, it spreads with startling rapidity from
one end of the long rows to the other. The phylloxera which has spoilt
the French vineyards is a root-feeding form that attacks the vine, and
kills or maims the plant terribly, by sucking the vital juices on their
way up into the fresh-forming foliage. The 'American blight' on apple
trees is yet another member of the same family, a wee creeping cottony
creature that hides among the fissures of the bark, and drives its very
long beak far down into the green sappy layer underlying the dead outer
covering. In fact, almost all the best-known 'blights' and
bladder-forming insects are aphides of one kind or another, affecting
leaves, or stalks, or roots, or branches.
It is one of the most remarkable examples of the limitation of human
powers that while we can easily exterminate large animals like the wolf
and the bear in England, or the puma and the wolverine in the settled
States of America, we should be so comparatively weak against the
Colorado beetle or the fourteen-year locust, and so absolutely powerless
against the hop-fly, the turnip-fly, and the phylloxera.
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